The present invention relates to a method for refurbishing turbine vanes, and more particularly, to a method of refurbishing in which an alloy blend powder corresponding to the base cobalt material of the turbine vane platform and the base nickel material of the replacement turbine vane airfoils is laser clad to the gaspath surfaces of turbine vane platforms, thereby improving the mechanical and coating properties of the refurbished vane.
Components of gas turbine engines, especially those positioned within the hot section of the engine, are exposed to a harsh operating environment. Extreme operating temperatures, accompanied by repeated temperature cycling during engine warm-up, operation and cool-down can quickly deteriorate engine components. These components include HPT (high-pressure turbine) vane segments which can become damaged or worn such that they require repair, i.e., refurbishment, or replacement.
A turbine engine vane segment is typically comprised of an outer and inner platform, between which one or more airfoils are positioned. The airfoils are either cast as a single unit with one or both of the platforms or are separately welded or brazed to the platforms in the form of a component assembly. Some turbine vanes are complex castings, comprising two, three or more airfoils integrally cast to the inner and outer platforms. Another form of turbine vanes is paired assemblies. A paired assembly is a vane in which a single airfoil is integrally cast between two platforms. Two of these castings are brazed or welded along mate face joints to create a doublet vane assembly.
Conventional airfoil replacement procedures involve separating the platforms by cutting the airfoils therefrom. This procedure retains a stub on each platform where the airfoils are cut out. The replacement airfoils are then typically welded to the stubs using electron beam (EB) welding techniques. Because the new airfoils must be positioned on the existing stubs, the positioning of the new airfoils is restricted. It is also extremely difficult to EB weld nickel airfoils to cobalt platforms, or to even EB weld nickel airfoils to nickel platforms. Also, because airfoil stubs are retained, complete refurbishment of the platform gaspath surfaces by an automated process is not possible. The irregularly contoured stub protruding from each platform requires that brazing and contouring of the platforms be done by hand. It is desirable, however, to automate as many refurbishment operations as possible in order to minimize repair prices and time.
Airfoil positioning within the vane, i.e., location on the vane segment platform, might require adjustment during vane refurbishment. For example, an adjustment to the nozzle opening area between adjacent vanes (hereinafter the "class area") may be required. For example, advances in material science often provide improved materials for use as airfoil members, and may provide airfoils having sizes and aerodynamic properties which differ from those used in existing vanes.
Components in gas turbine engines are air cooled and are fabricated from expensive materials. These components are also costly to assemble. As a result, it is desired to be able to efficiently repair the damage, while providing for upgraded components and materials within each vane, such that as much of the original materials as possible can be reused.
The concept of improving a turbine vane assembly by upgrading the alloy from which it is cast is known in the industry. As the ability to cast more complicated shapes from advanced high strength alloys has improved, turbine engine manufacturers implement these alloys on vanes having new configurations.
For example, General Electric Corporation produced first stage turbine vanes for LM1600 engines using X40 (cobalt) airfoils mated to X40 platforms. These vanes were subsequently manufactured using MA754 (nickel) airfoils with Mar M 509 (cobalt) platforms. The current configuration LM1600 vane uses Rene N5 airfoils (single crystal nickel) with DS Rene 142 platforms (directionally solidified nickel).
Another example is General Electric F404 first stage turbine vanes. The original F404 vanes had MA754 airfoils mated to Mar M 509 platforms. The F404 vanes were subsequently manufactured using N5 airfoils with DSR142 platforms.
Directionally solidified precipitation hardening nickel-based superalloys exhibit superior mechanical properties when compared with typical equiax structure cobalt based superalloys or equiax nickel based superalloys. As such, they are a logical choice to be used as replacement airfoils during refurbishment of a turbine vane. Problems arise, however, when a nickel based superalloy vane is mated with a cobalt based superalloy platform. In the past, vanes which were manufactured with this configuration, such as LM1600 or F404 vanes, did not have the airfoil surfaces coated. However, current engine operating conditions are so severe that bare alloy cannot provide adequate service life. It is therefore essential that all gaspath surfaces of the turbine vanes receive a protective coating to prevent oxidation and corrosion.
The preferred coating is an aluminide or precious metal (platinum) aluminide that may be applied by diffusion, overlay, or other means. The surface chemistry of the gaspath surfaces of the platforms must be compatible with the airfoils in order to achieve uniform coating (microstructure and properties) on the entire vane assembly gaspath. A cobalt based superalloy platform will coat at a different rate than a nickel based superalloy airfoil causing a nonuniform coating, rendering the gaspath surfaces non-compatible for coating purposes.
It is possible to clad cobalt based superalloy platforms with a straight nickel based superalloy when the vane is being refurbished using nickel based superalloy airfoils. However, the differing coefficients of thermal expansion along with differing yield strengths between the nickel superalloy cladding and the cobalt superalloy substrate cause significant cracking problems during the repair process and also during subsequent engine operation.
It is desirable, therefore, to provide a cladding material which can be applied to the surface of a cobalt based superalloy vane platform during vane refurbishment so as to allow the use of nickel based superalloy airfoils and to improve the mechanical properties of the vane while minimizing vane cracking during refurbishment and engine operation, and which is compatible with the aluminide coating. It is further desirable to refurbish the vane such that subsequent refurbishment is simplified.